The 20th episode of Gene Roddenberry’s science fiction series Star Trek airs on NBC. Percy Rodriguez and Elisha Cook guest star. 
The 20th episode of Gene Roddenberry’s science fiction series Star Trek airs on NBC. Percy Rodriguez and Elisha Cook guest star.
The 21st episode of Gene Roddenberry’s science fiction series Star Trek airs on NBC. Harry Townes guest stars.
The 22nd episode of Gene Roddenberry’s science fiction series Star Trek airs on NBC. Ricardo Montalban guest stars as Khan in the episode that sets up the events of 1982’s movie Star Trek II: The Wrath Of Khan.
The 23rd episode of Gene Roddenberry’s science fiction series Star Trek airs on NBC. David Opatoshu and Gene Lyons guest star.
The 24th episode of Gene Roddenberry’s science fiction series Star Trek airs on NBC. Jill Ireland guest stars.
The 25th episode of Gene Roddenberry’s science fiction series Star Trek airs on NBC. The Horta guest stars.
The 26th episode of Gene Roddenberry’s science fiction series Star Trek airs on NBC. John Colicos guest stars in the first Star Trek episode to introduce the Federation’s arch rivals, the Klingons. 
The 27th episode of Gene Roddenberry’s science fiction series Star Trek airs on NBC. The episode has been plagued by both script issues and an unusual but key piece of late re-casting.
The 28th episode of Gene Roddenberry’s science fiction series Star Trek airs on NBC. Joan Collins guest stars in an episode written by legendary science fiction writer Harlan Ellison; rewrites performed on the script prior to filming later become a major point of contention between Ellison and Roddenberry (and, by extension, between Ellison and Desilu Studios/Paramount).
The 29th episode of Gene Roddenberry’s science fiction series Star Trek airs on NBC. This episode concludes the series’ first season, and introduces numerous “fascinating” differences between human and Vulcan physiology.
The 30th episode of Gene Roddenberry’s science fiction series Star Trek premieres on NBC, kicking off the show’s second season with its first-ever glimpse of Spock’s home planet, Vulcan, as well as one of the best-known (and most parodied) fight scenes in the history of television, accompanied by some of the best-known TV soundtrack music ever.
The 31st episode of Gene Roddenberry’s science fiction series Star Trek premieres on NBC. The Enterprise once again trumps mythology with technology after encountering a powerful entity who gives the Enterprise a big hand.
The 32nd episode of Gene Roddenberry’s science fiction series Star Trek premieres on NBC. Captain Kirk continues his winning streak of logically talking sentient computers into self-destructing, impressing Mr. Spock to no end. Scotty becomes the latest senior Enterprise officer to die in the line of duty, and by the end of the episode becomes the latest senior officer to miraculously come back to life.
The 33rd episode of Gene Roddenberry’s science fiction series Star Trek premieres on NBC. This is the first appearance of the “Mirror universe” plotline that recurs through the Deep Space Nine, Enterprise, and Discovery spinoff series, and remains a fan favorite. Barbara Luna guest stars.
The 34th episode of Gene Roddenberry’s science fiction series Star Trek premieres on NBC. Once again, the Enterprise crew dismantles a machine maintaining a superstitious belief system on a backward world. David Soul (Starsky & Hutch) guest stars.
The 35th episode of Gene Roddenberry’s science fiction series Star Trek premieres on NBC. In one of the series’ most critically-acclaimed episodes, the Enterprise goes up against a monstrous device designed to devour entire worlds, and Commodore Matt Decker goes nuts.
The 36th episode of Gene Roddenberry’s science fiction series Star Trek premieres on NBC. It’s the only episode of the original series themed to a specific holiday (in this case, Halloween, in which it airs in close proximity), complete with a “giant” black cat, a Macbeth-esque trio of witches, and a script by Psycho novelist Robert Bloch.
The 37th episode of Gene Roddenberry’s science fiction series Star Trek premieres on NBC. Roger C. Carmel returns – the only recurring singular villain character of the original series – in the role of interplanetary rogue Harry Mudd.
The 38th episode of Gene Roddenberry’s science fiction series Star Trek premieres on NBC. Glenn Corbett guest stars as Zefram Cochrane, the man who invented warp drive (and shouldn’t, in the 23rd century, still be alive), a portrayal very different from what is shown in future Star Trek series and movies.
The 39th episode of Gene Roddenberry’s science fiction series Star Trek premieres on NBC. Jane Wyatt and Mark Lenard guest star as Spock’s parents in their first appearance in the Star Trek mythology (in fact, their only appearance prior to the animated series or the movies, despite the characters’ prominence), and Kirk gets into a knife fight trying to salvage interplanetary diplomacy. This episode’s depiction of Vulcan/Andorian unease would become a significant element of the later Star Trek TV spinoff Enterprise.
The 40th episode of Gene Roddenberry’s science fiction series Star Trek premieres on NBC. Julie Newmar takes a break from the Batcave to guest star in an episode focusing on the prime directive (and Kirk’s typical willingness to break with that most sacred of Federation laws).
The 41st episode of Gene Roddenberry’s science fiction series Star Trek premieres on NBC. The crew of the Enterprise contracts a disease that ages them rapidly toward death.
The 42nd episode of Gene Roddenberry’s science fiction series Star Trek premieres on NBC. Kirk grapples with a killer alien entity that he first encountered early in his Starfleet career.
The 43rd episode of Gene Roddenberry’s science fiction series Star Trek premieres on NBC. Robert Bloch, writer of the novel that inspired Hitchcock’s Psycho, returns with a script recasting Jack the Ripper as an extraterrestrial entity wreaking havoc down through the ages (with an airdate just before Christmas!).
The 44th episode of Gene Roddenberry’s science fiction series Star Trek premieres on NBC. This early script by writer David Gerrold becomes the best-loved episode of the entire series (according to many a fan poll), introducing the rapidly-reproducing, rapidly-eating, Klingon-sniffing Tribbles to the Star Trek universe. This is their only appearance in the original series, though they reappear in two animated series and Deep Space Nine years later.
The 45th episode of Gene Roddenberry’s science fiction series Star Trek premieres on NBC. Kirk, Chekov and Uhura find themselves drafted into playing deadly games for the amusement of unfathomable aliens; naturally, Kirk has a female “drill thrall” and introduces her to his own brand of inter-species relations.
The 46th episode of Gene Roddenberry’s science fiction series Star Trek premieres on NBC. Kirk and Spock gang up on a society who has taken a piece of outdated Earth literature and turned it into a blueprint for a blood-soaked society. (As heavy as that sounds, much hilarity ensues.) Vic Tayback (Alice) guest stars.
The 47th episode of Gene Roddenberry’s science fiction series Star Trek premieres on NBC. The Enterprise crew goes up against a psychedelic giant microbe (actually one of the most impressive effects of the original series).
The 48th episode of Gene Roddenberry’s science fiction series Star Trek premieres on NBC. Though it will become best remembered for the furry horned creature that attacks Kirk, the episode is actually a surprisingly hawkish (for Star Trek) allegory to the Vietnam War written by Roddenberry himself.
The 49th episode of Gene Roddenberry’s science fiction series Star Trek premieres on NBC. Diana Muldaur, who will later become a one-season regular on Star Trek: The Next Generation in the 1980s, makes the first of two classic Trek guest appearances.